The goals of this project are to characterize and use a new animal model for the measurement of outer hair cell (OHC) motility in vivo. The electrical stimulation of the cochlea with constant current acoustic frequency sinusoids results in electrically-evoked otoacoustic emissions (EEOAE) that can be measured in the ear canal. Placement of an electrode on the round window membrane of the cochlea generates the EEOAE and is non invasive to the inner ear. There are three Specific Aims. In Aim 1, the relationship of the parameters of the stimulation, particularly the position of the stimulation electrodes, to the EEOAE that is produced will first be determined. The goal is to show that the measured EEOAE bears a quantitative relationship to a particular population of OHCs. We will also determine the EEOAE produced from current injected into scala media, in order to relate the data of the new extracochlear stimulation method to the published literature where intracochlear current delivery has been used. In further experiments, this goal will also be achieved by altering and eliminating a population of hair cells with ototoxic drugs. A morphological assessment of the toxic effects of the drug treatment on OHCs will be performed. We will coordinate these studies with those of Project 2 (Aim 1) where the psychophysical percept of electrical stimulation is determined. In Aim 2, the goal is to determine whether the linearity of the EEOAE bears a predictable relationship to the known properties of electromotility obtained from in vitro studies. The quality and significance of in vivo electromotility is not known and thus this study should determine how electromotility contributes to cochlear nonlinear responses. This study is coordinated with Project 3 (Aim 3) where the influence of efferent neural activation on the EEOAE is explored. The measurement of the EEOAE not only provides information about the basic properties of electromotility in vivo but could also serve as the basis of a human hearing test determining the presence and health status of cochlear OHCs in future work.